A common example of a transmission line carrying radio frequency (RF) signals is a coaxial cable connecting an antenna to a voltage-sensitive equipment such as a receiver or transmitter. Often unprotected and exposed to the atmosphere, antennas and the transmission lines connecting the antennas to equipment are subject to high energy transient voltages or surges caused by lightening and, consequently, so too are the transmitters and receivers. Most such equipment have solid state devices at their input and/or output circuits. Since solid state devices have low breakdown voltages, at least as compared to vacuum tubes, a voltage surge can cause substantial damage to the equipment.
In order to protect voltage-sensitive equipment from damage caused by high voltage surges, a surge suppressor, sometimes also referred to as a surge arrestor or protector, is inserted between a transmission line and the equipment where the transmission line may be exposed to induced surges. The arrestor functions to shunt or discharge to ground high voltage transient signals. Several different types of discharge devices are available. An air gap device has two closely-spaced conductors, separated by an air gap. When a voltage differential across the gap reaches a sufficiently high level, the air ionizes and begins to conduct, thus discharging the surge to ground. An air gap device does not provide adequate protection for solid state components. A relatively high-voltage differential is required before it will conduct. It also has a slow response time. Furthermore, the conductors breakdown after a few lightening strikes. Thus, air gaps tend to be less reliable than other discharge devices.
Gas discharge tubes, on the other hand, tend not to deteriorate from frequent discharges. Relatively small, compact gas tubes are also able to handle large currents. However, at the relatively low discharge voltage thresholds desired for equipment with solid state devices, a gas discharge tube can be triggered by peak voltages in an RF signal, thus momentarily interrupting the signal. At higher discharge voltage thresholds, gas discharge tubes may not provide adequate protection. More importantly, gas discharge tubes have relatively slow response times. Therefore, some fast rising, high voltage spikes may not be discharged. Solid state discharge devices such as sidactors, avalanche type diodes and metal oxide varistors, on the other hand, have comparatively low voltage thresholds and quick response times for discharging transient voltages. However, solid state discharge devices have, as compared to similarly sized gas discharge tubes, low current handling capacity. Very large solid state devices are required for discharging large current surges, making them impractical to use in many applications.
All discharge devices have associated with them parasitic capacitances that tend to load the transmission line in the radio frequency range, thus attenuating the desirable RF signals. In a radio frequency application, therefore, a discharge device is preferably coupled to the transmission line in a manner that provides adequate and reliable protection and avoids significantly degrading or attenuating RF signals on the transmission line. Several examples of circuits for coupling various types of discharge devices, including an air gap discharge tube and a gas discharge tube, to RF transmission lines are disclosed in U.S. Pat. Nos. 4,359,764, 4,409,637 and 4,554,608 of Block.
Block's RF surge suppressor units include two connectors, two conductors, at least one discharge device and a capacitor, all of which are matched to the transmission line to pass RF signals. Segments of an RF transmission line are connected to the two connectors, and the two conductors interconnect the connectors. The discharge device is connected between one of the conductors and ground or between the two conductors, and the capacitor is inserted in line with one of the conductors. The impedances of the conductors, the capacitor and the discharge device are tuned so that the entire circuit matches the characteristic impedance of the transmission line at the desired operational radio frequencies, thus assuring minimal loss or attenuation of the RF signals travelling through the unit along the transmission line. The capacitor blocks direct current voltages commonly associated with transients caused by lightening from travelling through the surge arrestor. Once voltage on the transmission line builds to the breakdown voltage of the discharge device, the discharge device shunts the transient signal to ground.
In a surge arrestor unit disclosed in U.S. Pat. No. 5,122,921 of Koss, a capacitor is placed in line with one of the conductors of RF coaxial transmission line to block flow of direct current, just as in the circuits of Block. On the side of the capacitor to be connected to an antenna, a gas tube discharge device and a choke are connected in parallel with each other and between the inner conductor and the outer conductor of the RF transmission line (which is grounded). To the side of the capacitor to be connected with the voltage sensitive equipment is a resistor connected between the inner and outer conductors of the transmission line. The choke passes small direct current transients to ground. The capacitor, on the other hand, passes RF signals but not direct current signals. The choke creates a back emf when the speed and magnitude of a transient surge is sufficiently high to create a voltage that causes the discharge device to breakdown and conduct, thus protecting the capacitor. The resistor discharges any voltages on the equipment side of the capacitor.
The surge suppressor units of Block and Koss suffer from several problems. First, fast rising transients tend to pass through the in-line capacitor and to the equipment before the discharge device begins to conduct. Second, high voltages across the capacitor tend to cause a reverse transient flow of current from the equipment. Third, surge suppressors fabricated using conventional materials and methods, like those of Block, require that each unit be manually tuned during assembly. The impedances inherent in the conductors in each unit depend on the physical geometry of the conductors and the other components within the unit. The impedances will tend to vary between units due to the difficulty of precisely reproducing the physical layout of the units. Thus, tuning not only must take into account the variations in the inherent impedance of the discharge devices, but also the natural variations in the parasitic impedances of the conductors. Since surge suppressors such as those of Block rely on the parasitic reactances of the conductors to match the discharge device to the transmission line, careful tuning is critical for their satisfactory performance. Fourth, breakdown and conduction of the discharge device in both Block and Koss interrupts RF signal flow along the transmission line since the capacitance of the discharge device drops out of the circuit when it conducts. Fifth, the capacitors totally block the flow of direct current along the RF transmission line. However, RF transmission lines are now being used to deliver power to equipment located on an antenna, such as down converters and amplifiers. Thus, the surge suppressors of Block and Koss cannot be used in these RF transmitting and receiving systems.